1. Field of the Invention
The present invention relates to a low hydrogen overvoltage cathode for the electrolysis of water or an aqueous alkali metal chloride such as aqueous sodium chloride, and also to a process for producing the low hydrogen overvoltage cathode.
2. Description of the Related Art
Industrial electrolysis of water or an aqueous alkali metal chloride consumes a large amount of electric power, so that various energy saving techniques are being developed for industrial electrolysis procedures. "Energy saving techniques" means techniques which result in a substantial decrease of the electrolysis voltage which techniques can include decreasing the theoretical electrolysis voltage, solution resistance, diaphragm resistance, cathode overvoltage and anode overvoltage. In particular, the mentioned overvoltages, which largely depend on the electrode material and the electrode surface state, have attracted the attention of many research scientists, and many developments have been made in this area.
For instance, in the ion exchange process for sodium chloride electrolysis, a decrease of the anode overvoltage has been actively studied. Consequently, anodes have been developed which do not involve problems regarding anode overvoltage; such anodes are in wide use industrially.
Many proposals have also been made regarding low hydrogen overvoltage cathodes, namely active cathodes which can have their hydrogen overvoltage lowered by 200-250 mV in comparison with a conventional iron cathode exhibiting a hydrogen overvoltage of 400 mV. For example, a hydrogen absorbing alloy or a platinum group metal oxide has been deposited on an electrode base material surface (Japanese Patent Laid-Open Publications 59-25940 and 6-146046). Further, a coating layer of an alloy of a transition metal such as iron, cobalt and nickel, tungsten or molybdenum has been formed by plating the same on an electrode base material surface (Japanese Patent Publication 40-9130). However, the electrodes having a hydrogen absorbing alloy or a platinum group metal oxide deposited thereon use an expensive material, which results in high cost, whereas while the latter electrodes covered with an alloy of a transition metal, etc., can be produced at low cost, they are not sufficient in reducing the hydrogen overvoltage. Thus, both types of electrodes still involve problems.
To improve electrodes plated with an alloy of iron, cobalt, nickel or molybdenum, a water-soluble polyamine has been added to the alloy plating bath (Japanese Patent Laid-Open Publication 55-65376). However, this involves disadvantages in that the polyamine is soluble only over a narrow pH range which makes control of the plating bath difficult on an industrial scale. Further, the decrease of the hydrogen overvoltage is still insufficient.
Most of the active cathodes to date comprise an electrode base material and a catalyst layer of a specific composition formed thereon to decrease the hydrogen overvoltage. The coating layer is formed in various ways. For example, a catalytic substance can be electrically deposited by wet plating from a bath containing a dispersed active substance or containing a dissolved metal salt as disclosed in the aforementioned patents; a catalytic metal substance in a molten state can be directly sprayed onto a base material (Japanese Patent Laid-Open Publication 61-41786); a metal salt solution can be applied onto a base material, dried, and subjected to reduction or other treatment to form a catalytic substance layer (Japanese Patent Laid-Open Publication 61-295386); etc. However, in the wet plating method the alloy composition for coating is limited due to differences in electrodeposition potentials which is a disadvantage. Further, the composition of the active substances or the metal components in the plating bath tend to change over the time of plating, requiring strict control of the bath to obtain a homogeneous alloy layer in a stable manner. On the other hand, in the last two methods, alloy formation is difficult with elements having a large difference in vapor pressure because of the high temperature treatment required for coating, and an amorphous or fine crystalline structure of high performance cannot readily be obtained because of enhanced crystallization in the high temperature treatment, which is disadvantageous. To avoid crystallization, a sputtering method has been proposed (Japanese Patent Laid-Open Publication 7-268676). However, the sputtering method still has the problem that the film formation rate is low.